The present invention relates to photomultiplier for a fluorescent spectroscopic analyzer or the like based on a time-correlated single photon counting (SCP) or the like, and particularly relates to a photomultiplier in which weak light such as fluorescent light incident upon a photoelectron emission surface or a photocathode is converted into an electrical current corresponding to the intensity of the weak light.
In an ordinary photomultiplier, photoelectrons or primary electrons, which are emitted from the photocathode by light incident to the photocathode, are multiplied a large number of times by the secondary electron emission surfaces of plural dynodes so that an electrical current corresponding to the intensity the light is outputted from an anode.
FIG. 1 shows a sectional view of a conventional head-on-type photomultiplier 51 comprising the photocathode 4, a first focusing electrode 52, a second focusing electrode 53, a flat plate electrode 6, a first dynode 7 to a twelfth dynode 18 having secondary electron emission surfaces, an anode 19 and a cylindrical housing 2 containing the foregoing components therein. One end of the housing 2 is closed by a transparent light incident plate 3 and the other end of the housing is closed by a stem and covered with a plastic cap 20. The inside surface of the light-incident plate 3 is slightly curved at a radius of curvature, which is 55 mm or the like. The photocathode is curved along the inside surface of the light-incident plate 3 and made of a conventional photoelectron emission material such as a bialkali compound having a composition of K-Sb-Cs and a compound having a composition of Na-K-Sb-Cs (the S number is 20). The dynodes 7 to 18 are made of a nickel material. The inside surfaces of the dynodes are provided with the secondary electron emission surfaces made of an alkali antimonide having a composition of K-Cb-Cs, and are coated with a film of SbCs. The secondary electron emission surfaces of the dynodes are not shown in FIG. 1 except that of the first dynode 7. The first and the second focusing electrodes 52 and 53 are cylindrically shaped, and are provided between the photocathode 4 and the first dynode 7 so that photoelectrons or primary electrons emitted from the photocathode are converged to the first dynode. The tops of the first and the second focusing electrodes 52 and 53 are open. The central portion of the bottom of the first focusing electrode 52 has an opening 54 in which the second focusing electrode 53 is inserted. The central portion of the bottom of the second focusing electrode 53 has an opening 55 through which the primary electrons pass. The flat plate electrode 6 supports the first and the second focusing electrodes 52 and 53 so as to electrically separate the photocathode 4 from the dynodes 7 to 18 and the anode 19, and has a center opening 24 through which the primary electrons pass. The openings 54, 55 and 24 of the first and the second focusing electrodes 52 and 53 and the flat plate electrode 6 are concentrically provided to the housing 2. The photocathode 4, the first focusing electrode 52, the second focusing electrode 53, the flat plate electrode 6, the dynodes 7 to 18 and the anode 19 are connected to corresponding connection pins K, G, G1, DY1 to DY12 and P through stem pins and lead wires which are not shown in FIG. 1.
FIG. 2 shows the state of connection of the connection pins K, G, DY1 to DY12 and P of the photomultiplier 51 and an external circuit 37 which has sockets S14, S15, S1 to S13 corresponding to the connection pins. The socket S14 is connected to a power supply (which is not shown in the drawings) for applying a voltage (-H V). The sockets S15, S1 to S12 are connected to the power supply through bleeder resistors R1 to R16 and capacitors C1 to C9. One terminal of the bleeder resistor R16 is grounded. The capacitors C1 to C9 connected parallely with the bleeder resistors R10 to R16 are provided to keep the sockets S7 to S12 at predetermined potentials. The socket S13 is connected to a coaxial cable CBL. Since the external circuit 37 is used for detecting weak light, the number of incident photons of which is so small that the photons can be detected separately from each other, the socket S13 for taking out an output signal from the photomultiplier 51 is connected to the coaxial cable CBL through which the output pulse signal can be accurately transmitted.
When the photomultiplier 51 is connected to the external circuit 37, the photocathode 4 of the photomultiplier is kept at the lowest potential of -H V (e.g., -2,500 V) through the pin K. At that time, the potentials on the first focusing electrode 52 and the dynodes 7 to 18 are kept sequentially higher than the lowest potential on the photocathode 4, through the pins G, DY1 to DY12. The anode 19 is kept at the ground potential through the pin P, and the second focusing electrode 53 is kept at the same potential as the seventh dynode 13 through the pin G1.
A time-correlated single photon counting (SPC) is often used for a fluorescent spectroscopic analyzer or the like so as to measure weak short-lived fluorescent light or the like. In the fluorescent spectroscopic analyzer, an exciting light pulse EX having a sufficiently small width as shown in FIG. 3(A) is irradiated upon a sample such as a living body substance and a semiconductor to transit the molecules of the sample from the ground state to an excited state depending on the energy of the exciting light pulse. After that, the excited molecules go back to the ground state from the excited state to emit fluorescent light having a wavelength corresponding to an energy gap between the excited state and the ground state. In the time-correlated single photon counting, the intensity of the exciting light pulse EX is preset at a reduced level so that only single photon SP of the fluorescent light is detected within an observation time, whereby the single photon SP is emitted at a time point t.sub.2 as shown in FIG. 3(C) after the sample is excited by the exciting light pulse EX at a time point t.sub.1 as shown in FIG. 3(A). The probability of the emission of the single photon SP reaches a maximum when a very short time has elapsed since the time point t.sub. 1 at which the molecules are excited by the exciting light pulse EX. The probability decreases nearly exponentially with the lapse of time from the maximum. In the time-correlated single photon counting, the exciting light pulse EX is repeatedly irradiated upon the sample to repeatedly emit the single photon SP as shown in FIG. 3(C), thereby to determine the frequency .alpha. of the single photon with respect to the time of the emission thereof and obtain a fluorescent light damping curve CV.sub.o (t) indicating the time characteristic of the fluorescent light as shown in FIG. 3(B).
FIG. 4 shows a schematic view of an apparatus for measuring the weak light such as fluorescent light using the time-correlated single photon counting. In this apparatus, the time of the emission of the single photon does not fluctuate in accordance with the probability but is predetermined, so that when the measurement of the output signal from the photomultiplier of the apparatus is repeated by repeating irradiation of the single photon upon the photomultiplier, there would ideally appear a distribution in which a frequency corresponding to the number of the repetition is present only at a certain time point, in place of the fluorescent light damping curve.
FIG. 5(A) shows a start signal STT applied to a time-to-amplitude converter (TAC) 60 shown in FIG. 4, and FIG. 5(B) shows the output signal from the photomultiplier 51. FIG. 6(A) is a diagram for explaining a theshold value for the output signal from the photomultiplier 51, and FIG. 6(B) is a diagram for explaining a procedure of detecting only a light pulse current out of the output signal from the photomultiplier 51 based on the threshold value as determined in FIG. 6(A).
In the measuring apparatus shown in FIG. 4, the fluorescent light from the actual sample is not used but the weak light corresponding to the fluorescent light, which is generated by a pulse generator 62, an optical fiber 63 and a filter 64, is used. Therefore, the time of the emission of the single photon SP does not fluctuate in accordance with the probability thereof, but is predetermined. The apparatus is controlled by a computer 58 which is connected to a multichannel analyzer (MCA) 59. The time-to-amplitude converter 60 is connected to the multichannel analyzer 59. Time-to-amplitude converter 60 is supplied with the start signal STT as shown in FIG. 5(A) and measures the time difference between the generation of the start signal STT and that of a stop signal STOP as described hereinafter. If two output signals, that is, two pulse currents are outputted from the photomultiplier 51 and then two stop signals STOP are outputted from a constant-fraction discriminator (CFD) 66 per start signal STT, the time-to-amplitude converter 60 measures only the time difference between the generation of the start signal and that of the prior stop signal, disregarding the posterior stop signal. A delay circuit 61 applies the start signal STT to the time-to-amplitude converter 60 after a predetermined delay time it lapsed from a time at which the light is emitted from the pulse generator 62. For example, the delay circuit 61 is set so that the predetermined time is about 200 nanoseconds.
The pulse generator 62 includes a light emission diode (not shown in the drawings) for emitting light of 410 nanometers in wavelength. The light emitted from the light emission diode is guided to the filter 64 through the optical fiber 63. Before the light is entered into the photomultiplier 51, the filter 64 decreases the quantity of the light to create such a state (which is hereinafter called the SPE state) of single photoelectron event that only the photon can be detected in the photomultiplier 51 within the observation time. As a result, the single photon SP is irradiated upon the photomultiplier 51 after the lapse of the predetermined time from the time at which the light is generated by the pulse generator 62.
As mentioned above, the predetermined potentials are applied to the electrodes of the photomultiplier 51 from the external circuit 37 so that the photoelectrons or primary electrons are emitted from the photocathode 4 by the weak light incident upon the photomultiplier. The primary electrons emitted from the photocathode 4 are converged by the first and the second focusing electrodes 52 and 53 and reach the first dynode 7 through the opening 55 of the second focusing electrode and the opening 24 of the flat plate electrode 6. Secondary electrons are emitted from the secondary electron emission surface 22 of the first dynode according to the incident primary electrons to the first dynode. The secondary electrons reach the secondary electron emission surfaces (which are not shown in the drawings) of the second dynode 8 to the twelfth dynode 18 so that multiplication is performed through each secondary electron emission surface. As a result, the output signal is outputted in the form of an electrical current from the anode 19 to the external circuit 37.
Since the light incident upon the photocathode 4 is so weak as to create the SPE state, the output signal from the anode 19 consists of pulse currents PC1, PC2 and PC3 as shown in FIG. 5(B). The pulse current PC1 is a main pulse current and outputted from the photomultiplier 51 after a lapse of a time which it takes for the electrons to transit in the photomultiplier from the time of the irradiation of the single photon SP upon the photocathode 4.
The light-incident plate 3 of the photomultiplier 51 is covered with a black tape or the like except the 10-mm-diameter circle area of the plate to which a light is actually incident, in order to prevent the light from reaching an area of the photocathode 4 except the 10-mm-diameter circle area thereof.
The output signal, that is, the pulse current which is outputted from the external circuit 37, is amplified by an amplifier 65 and then supplied to the constant-fraction discriminator 66. The constant fraction discriminator 66 outputs only the pulse current larger than a predetemined threshold value LLD among the pulse currents from the amplifier 65. The LLD is set at a pulse-height at which the distribution of pulse-heights is minimum as shown in FIG. 6(A), and therefore the other pulse currents PC2 and PC3 are removed as noises caused by the dark currents of the photomultiplier 51. Accordingly, only the pulse current PC1 whose height is larger than the threshold value LLD is detected as a light pulse current.
When the light pulse current whose height is larger than the threshold value LLD is detected as mentioned above, the constant fraction discriminator 66 outputs the stop signal STOP to the time-to-amplitude converter 60 so that the converter does not accept the other following light pulse currents. The start signal STT from the pulse generator 62 is inputted to the time-to-amplitude converter 60 through the delay circuit 61 prior to an input of the stop signal STOP to the converter 60. The time-to-amplitude converter 60 recognizes in response to the stop signal STOP supplied from the constant-fraction discriminator 66 that the first light pulse current is generated for a start signal STT. The converter 60 measures the time tt which lapses from the generation of the start signal STT to that of the stop signal STOP.
Since the start signal STT is inputted to the time-to-amplitude converter 60 from the pulse generator 62 at a certain time and the stop signal STOP must ideally be outputted from the pulse generator after the lapse of a prescribed time from the time of the generation of the light from the pulse generator, the time tt must be constant. However, the time tt fluctuates because the orbits of the primary and the secondary electrons in the photomultiplier 51 are irregular.
When the time tt from the generation of the start signal STT to that of the stop signal STOP is measured by the time-to-amplitude converter 60, the result of the measurement is sent as a piece of measurement data to the multichannel analyzer 59 and the frequency .alpha. of the single photon for the time tt is increased by one in the computer 58.
FIG. 7 shows photon counting data obtained by repeatedly (100,000 times, for example) irradiating the single photon SP upon the photomultiplier 51 and supplying a plotter 57 with the photon frequency for the time tt from the generation of the start signal STT to that of the stop signal STOP. In FIG. 7, the time point tt of the highest photon frequency is shown as 0 nanosecond.
If the orbits of the primary and the secondary electrons in the photomultiplier 51 were not irregular, repeatedly mesured photon counting data should be detected as an ideal pulse current IP having a generation frequency corresponding to the number of the times of the repetition, only at a time point of 0 nanosecond as shown in FIG. 7. However, the orbits of the primary and the secondary electrons in the photomultiplier 51 are irregular, so that a main pulse current MP.sub.1 having a time fluctuation of full width at half-maximum FWHM1 as shown in FIG. 7 and a residual pulse current AP.sub.1 generated shortly after the generation of the main pulse current are practically detected. According to the conventional photomultiplier 51, it is understood from the photon counting data as shown in FIG. 7 that the full width at half-maximum FWHM1 of the single photon frequency corresponding to the main pulse current MP.sub.1 is in the range of 500 to 600 picoseconds and the residual pulse current AP.sub.1 is detected with a generation probability of 3 to 4% after about 15 to 20 nanoseconds from the detection of the main pulse current. The generation probability of the residual pulse current AP.sub.1 is calculated as the ratio of the total frequencies AR2 of single photon for the residual pulse current AP.sub.1 to those AR1 of single photon for the main pulse current MP.sub.1.
The distributions of the frequencies of single photons SP for the main and the residual pulse currents MP.sub.1 and AP.sub.1 as shown in FIG. 7 are the results of the detection of the pulse currents which is performed in a case where the time of the generation of the single photon SP is predetermined and therefore is not fluctuated. In the actual measurement of the fluorescent light, however, the single photon SP is incident to the photomultiplier 51 according to the time characteristic as shown in FIG. 3(B), that is, the fluorescent light damping curve CV.sub.o (t), so that the temporal change in the single photon frequency actually detected by the photomultiplier 51 can be predicted in accordance with the following time convolution CV(t) of the time characteristic or damping curve CV.sub.o (t') of the actual fluorescent light and the time fluctuation curve g(t'-t) of the main and the residual pulse currents MP.sub.1 and AP.sub.1. EQU CV(t)=.intg.CVo(t').multidot.g(t'-t)dt'
The time convolution is calculated by the computer 58 and simultaneously outputted as fluorescent light damping data CV.sub.1 (t) as shown in FIG. 7 to the plotter 47.
According to the conventional photomultiplier 51 shown in FIG. 1, the main pulse current MP1 has the time fluctuation of the full width at half-maximum FWHM1 which is in the range of about 500 to 600 picoseconds, and the residual pulse current AP.sub.1 is outputted with the generation probability of 3 to 4% and measured in addition to the main pulse current.
The residual pulse current AP.sub.1 has been recently thought to be generated due to the light feedback in which light emitted from the first dynode 7 proceeds to the photocathode 4 and returns to the first dynode. The present inventor et al have found out the rule that the time from the generation of the main pulse current MP.sub.1 to that of the residual pulse current AP.sub.1 is twice s long as the transit time of the primary electrons from the photocathode 4 to the first dynode 7. Since the transit time of the light from the photocathode 4 to the first dynode 7 in the light feedback is several hundred picoseconds which are much shorter than the transit time of the electrons, the above-mentioned rule could not exist if the residual pulse current AP.sub.1 were generated due to the light feedback. Paying attention to the fact that the time from the generation of the main pulse current MP.sub.1 to that of the residual pulse current AP.sub.1 is twice as long as the transit time of the primary electrons from the photocathode 4 to the first dynode 7, the present inventor et al have discovered that the residual pulse current is not generated due to the light feedback but generated due to the phenomenon that the secondary electrons emitted from the secondary electron emission surface 22 of the first dynode 7 proceed to the photocathode and returns to the first dynode as indicated by orbits G1, G2, G3, G4 and G5 as shown in FIG. 1.
FIG. 8 shows the distribution of energy of the secondary electrons which are emitted from the secondary electron emission surface 22 of the first dynode 7 when the primary electrons with the energy of 100 eV impinge on the secondary electron emission surface. It is apparent from FIG. 8 that the distribution of energy of the secondary electrons can be classified into three regions a, b and c. In the region a, the secondary electron is emitted with the energy of about 2 eV. In the region c, the secondary electron is emitted with slightly less energy than the primary electron. In the region a, the secondary electrons are ordinary secondary electrons newly emitted from the secondary electron emission surface 22. In the region b, some of the secondary electrons are newly-emitted ordinary secondary electrons and the others are primary electrons which have impinged on the secondary electorn emission surface 22 with a loss of a part of energy in the process of exchanging energy on the surface and thereafter has been non-elastically reflected from the surface. The secondary electrons which are the primary electrons nonelastically reflected from the secondary electron emission surface 22 as described above are called backscattered electrons. In the region c, the secondary electrons are primary electrons which have lost a very small quantity of energy on the secondary electron emission surface 22 and therefore has been nearly elastically reflected from the surface. The electrons which are nearly elastically reflected from the surface 22 as described above are called elastically reflected electrons.
The secondary electrons in the region a of the distribution of energy correspond to the main pulse current MP.sub.1 generated as shown in FIG. 9(A). The secondary electrons in the region b of the distribution of energy correspond to a main pulse current MP.sub.1 ' and a residual pulse current AP.sub.1 ' generated in a very short time after the main pulse current MP.sub.1 ' as shown in FIG. 9(B). In other words, the secondary electrons which are emitted as the ordinary secondary electrons in the region b of the distribution of energy and correspond to the main pulse current MP.sub.1 ', and the secondary electrons which are in the region b of the distribution of energy and emitted as the backscattered electrons correspond to the residual pulse current AP.sub.1 '. Since the backscattered electrons do not reach the photocathode 4, but change their directions and then returns to the first dynode 7, the residual pulse current AP.sub.1 ' corresponding to the backscattered electrons is generated in the very short time after the generation of the main pulse current MP.sub.1 '. However, the time-to-amplitude converter 60 of the apparatus measures only the time from the generation of the start signal STT to that of the stop signal STOP based on the first output signal, that is, the main pulse current MP.sub.1 ', and therefore the residual pulse current AP.sub.1 ' based on the backscattered electrons is not practically measured. The secondary electrons which are in the region c of the distribution of energy and are the elastically reflected electrons correspond to the residual pulse current AP.sub.1 as shown in FIG. 9(C). The elastically reflected electrons are emitted from the first dynode 7 with slightly less energy than that of the electrons incident upon the first dynode, so that the elastically refected electrons proceed to the vicinity of the photocathode 4 as indicated by the orbits G1 to G5 as shown in FIG. 7, change their directions in that vicinity and return to the first dynode. Accordingly, a pulse current based on the elastically reflected electrons is outputted from the anode 19 with a time lag which is nearly twice as long as the transit time of the electrons from the photocathode 4 to the first dynode 7 and the pulse current is measured as the residual pulse current AP.sub.1 as shown in FIG. 7. Since the elastically reflected electrons entail no main pulse current MP.sub.1, the time tt up to the generation of the stop signal STOP based on the residual pulse current AP.sub.1 is practically measured by the time-to-amplitude converter 60 of the measuring device.
The orbits G1 to G5 of the elastically reflected electrons are calculated through computerized simulation. It is assumed in the calculation that a distribution of the emitting angles of the elastically reflected electrons from the first dynode 7 depends on the incident angles of the primary electrons from the photocathode 4 to the first dynode 7 and that the elastically reflected electron is reflected in the same direction as the incidence of the primary electron with high probability.
FIG. 10 shows the distribution of the emitting angles of the elastically reflected electrons from the first dynode 7. It is apparent from FIG. 10 that the distributions AD.sub.0, AD.sub.1 and AD.sub.2 of the emitting angles of the elastically reflected electrons corresponding to the primary electrons impinging on the first dynode 7 at incident angles .theta. of 0.degree., 30.degree. and 45.degree. have their main directions at angles .theta. of 0.degree., 30.degree. and 45.degree..
The residual pulse current AP.sub.1 generated and measured as described above causes the accuracy of the analysis of photon counting data based on the main pulse current MD.sub.1 to be reduced and the calculation of the time convolution of the actual fluorescent light damping curve CV.sub.o (t) in FIG. 3(B) affords the fluorescent light damping data CV.sub.1 (t) shown in FIG. 7. Therefore, the actual fluorescent light damping curve CV.sub.o (t) cannot be accurately predicted and it is preferable to remove the residual pulse current AP.sub.1.
However, the elastically reflected electrons proceeding to the vicinity of the photocathode 4 return to the first dynode 7 with no obstacle to transit, and further the elastically reflected electrons are generated without being affected by materials of the secondary electron emission surface 22 of the first dynode 7 in the conventional photomultiplier 51, so that the photomultiplier has a problem that it is difficult to effectively suppress the generation of the residual pulse current AP.sub.1.